Hi Villa1, turned out to be something simple. went into the attic this evening and looked into the small tank, the ballcock was set to high and the water level was just lipping off the bottom of the overflow pipe, so when the heating came on and there was some expansion there was an immediate overflow, so i took a jugfull out and bent the bar of the ball cock and the water level ended up being about an inch under the overflow hole, so it seems to have sorted the problem. Thanks for the help. Hopefully thats the end of the problem, seems to be grand so far.